Wastewater treatment system and method of using same

ABSTRACT

A wastewater treatment system, comprising a tank and an elongate draft tube. The tank comprises a bottom and at least one partition wall extending short of a tank curved turning wall to form at least a pair of channels for movement of a volume of a liquid. The elongate draft tube is at least partially submerged beneath the liquid and is rotated about its longitudinal axis for orbitally moving the liquid through the at least a pair of channels of the tank in a fixed direction. A process for treating wastewater is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims priority to and thebenefit of U.S. patent application Ser. No. 11/340,976, filed on Jan.27, 2006, now U.S. Pat. No. 7,559,538 which is incorporated in itsentirety in this document by reference.

FIELD OF THE INVENTION

This invention relates to a wastewater treatment system. Moreparticularly, it relates to a system and method in which mixed liquor issubject to treatment in a deep aeration basin and to a system and methodin which mixed liquor is subject to treatment while being propelledaround an orbital, essentially closed circuit, tank.

BACKGROUND OF THE INVENTION

Presently, low speed aeration rotors are large diameter centrifugal pumprotors that operate horizontally at the air-liquid surface boundary,mixing air and water. In use, the rotor draws water from beneath therotor and sprays it horizontally over the water surface. The rotor alsoimparts a rotary motion to the body of water surrounding the rotor. Inoxidation ditch applications, the rotary motion imparted by the rotoralso forces the water in the ditch to circulate around the ditch. In oneexample of such a system, U.S. Pat. No. 3,510,110 to Kline, discloses anorbital system employing an elongated tank with central partition thatincludes a vertically-rotated surface aerator located at the end(s) ofthe partition wall for both aerating the sewage and circulating thesewage around the channels formed by the partition wall and the sides ofthe tank.

One example of an orbital system is sold under the trademark Carrousel®.An exemplified Carrousel® system, as with any typical oxidation ditch,has a basin that is shaped like a race track and has a central,longitudinally extending partition wall. The mixed liquor within theditch is oxygenated by at least one low-speed vertical shaft aerator,which ensures proper mixing while generating the horizontal velocity andturbulence necessary to prevent sludge settling in the circuit. In use,while the wastewater is circulating around the channel, micro-organisms,such as activated sludge, utilize the organic compounds, nitrogen andphosphorus contained in the waste. Depending on how the system isemployed, the circulation of the wastewater exposes the activated sludgeto oxygen-rich, i.e., aerobic and oxygen-depleted, i.e., anoxicconditions. In use, the low-speed, vertical shaft, turbine aeratorprovides the necessary oxygen to support biological utilization, whilealso keeping the biomass in suspension by driving the wastewater in aturbulent flow across the entire looped channel. To obtain the mostefficient level of nutrient removal, the power input is adjusted inrelation to the actual oxygen demand and load conditions, by varying thespeed and/or the submergence of each aerator. When the oxygen demand islow, aeration power can be further reduced by shifting the speed of theaerators, or by switching them off altogether.

The popularity of the conventional orbital systems is due primarily totheir relative cost-effectiveness, simplicity of design, ease ofoperation and maintenance, and excellent effluent quality. Theexemplified conventional orbital system can treat raw domestic water toEPA advanced secondary standards without primary clarifiers or effluentfilters. With extended aeration, it produces a stable water sludgerequiring little or no further processing prior to disposal. Theconventional systems can be designed to have a power turn-down to matchoxygen input to the mixed liquor to oxygen demand of the microbes actingto degrade the sewage, without loss of mixing and movement.

However, deep oxidation ditches and/or deep aeration basins (forexample, and not meant to be limiting, about or greater than 4.5 metersdeep) are sometimes beneficial because more matter can be processed in agiven amount of surface area. However, the suction effect ofconventional rotors is generally limited to about 6 meters in depth, andthe rotary motion in oxidation ditches or basins is generally limited toabout 4.5 meters in depth. In order to achieve a satisfactory flowvelocity in the basins, conventional orbital systems are designed with amaximum depth of about 4.5 meters.

One example of a system for driving fluids below the effective depthlimitations of the conventional rotors outlined above is disclosed inU.S. Pat. No. 4,869,818 to DiGregorio, et al. In this system, a radialflow submerged impeller is added to the same shaft that drives thesurface aerator so that mixed liquor in the lower portion of the orbitalchannels is pumped in the same direction as that mixed liquor pumped bythe surface aerator. Thus, the system urges movement of the fluid thatwould have been unaffected by the surface rotor and effectivelyalleviates certain depth restrictions in orbital tanks, which allows forthe use of deeper channels that require less concrete and less landspace. However, one will appreciate that adding an additional impellerthat extends deep within the basin also requires additional powerconsumption.

In another example for providing movement of the fluid located near thebottom of aerated basins of greater depth, draft tubes are provided tocooperate with the surface aeration rotor. In this example, the drafttube, which is essentially a large diameter pipe, is fixed to andextends from the bottom of the basin such that its distal end is spaceda distance from the bottom of the basis and its proximal end ispositioned below the surface aeration rotor. Here, the draft tube servesto concentrate the pumping action of the surface aeration rotor downtoward the bottom of the aerated basin. However, the obstructive bulk ofthe fixed draft tube greatly attenuates the rotary motion imparted bythe rotor, thus making the use of such a fixed draft tube impractical ina standard oxidation ditch and reduces the mixing effect in aeratedbasins. To overcome this limitation and to allow the use of a draft tubein a deeper ditch/basin system, conventionally practice requires atleast one horizontal flow mixer that is positioned within the lowerportion of the deeper ditch/basins. The additional required mixerrequires more complex machinery and expense as well as increase thepower consumption of the system.

From a dynamic point of view, the turbulent energy requirement of afluid for proper mixing is related to physical properties of the fluid,turbulence length scale created by a particular agitating device andturbulent intensity which has dominant effect on rate of decay ofkinetic energy. The turbulent intensity can be interpreted asfluctuating flow velocity and will affect the mass transfer of gas intoliquid on gas-liquid interface. An energy efficient aeration method hasto incorporate all these factors to produce the best mass transfermechanism. Balancing of these physical phenomena produces the mostenergy economical aeration as well as to produce favorable flowconfiguration for good mixing and solid suspension. Moreover, forpractical applications, maximum mechanical simplicity and minimummaintenance in operation are very important factors. The presentinvention is based on the above considerations.

SUMMARY

In one aspect of the present invention, a wastewater treatment systemcomprises a tank, which comprises at least a pair of channels formovement of a volume of a liquid, and an elongate draft tube, which isat least partially submerged beneath the liquid. In use, the elongatedraft tube is rotated about its longitudinal axis for orbitally movingthe liquid through the at least a pair of channels of the tank in afixed direction. In another aspect, the wastewater treatment systemfurther comprises a rotatable surface aerator that is at least partiallyimmersible into the liquid. In use, the surface aerator is rotated aboutthe longitudinal axis of the draft tube to move the liquid upward anddistribute it over the liquid surface.

Related methods of operation are also provided. Other systems, methods,features, and advantages of the wastewater treatment system will be orbecome apparent to one with skill in the art upon examination of thefollowing figures and detailed description. It is intended that all suchadditional systems, methods, features, and advantages be included withinthis description, be within the scope of the wastewater treatmentsystem, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description, serve to explain theprincipals of the invention. Moreover, in the figures, like referencenumerals designate corresponding parts throughout the different views.

FIG. 1 shows a top elevational view of an embodiment of the wastewatertreatment system of the present invention mounted thereon a conventionalorbital tank or basin.

FIG. 2 shows a partial cross-sectional view of a first embodiment of thewastewater treatment system of the present invention, showing arotatable elongate draft tube mounted proximate an end of a partitionwall of the orbital tank, and showing a fixed shaft that extends fromthe bottom of the tank, which rotatably supports the distal end of thedraft tube.

FIG. 3 shows an enlarged partial cross-sectional view of a cylindricaldraft tube, showing an insert mounted in a distal end of the draft tubesuch the effective inside diameter of the distal end of the draft tubeis less than the effective inside diameter of the proximal end of thedraft tube, and showing a blade mounted proximate an end portion of apartition wall.

FIG. 4 shows a partial cross-sectional view of a second embodiment ofthe wastewater treatment system of the present invention, showing atapered rotatable draft tube mounted proximate an end of a partitionwall of the orbital tank, wherein the effective inside diameter of thedistal end of the tapered draft tube is less than the effective insidediameter of the proximal end of the draft tube.

FIG. 5 shows a partial cross-sectional view of a third embodiment of thewastewater treatment system of the present invention, showing arotatable surface aerator connected to a driven shaft and showing arotatable elongate draft tube fixedly connected to a bottom portion ofthe surface aerator.

FIG. 6 shows a partial cross-sectional view of a fourth embodiment ofthe wastewater treatment system of the present invention, showing arotatable surface aerator connected to a driven shaft and showing arotatable draft tube connected to the driven shaft and spaced apredetermined distance from a bottom portion of the surface aerator.

FIG. 7A shows a partial cross-sectional view of a fifth embodiment ofthe wastewater treatment system of the present invention, showing arotatable surface aerator connected to a first driven shaft and arotatable draft tube fixedly connected to a bottom portion of thesurface aerator, and showing a rotatable submerged rotor or highefficiency turbine impeller mounted to a second driven shaft andpositioned therein the interior of the draft tube for moving liquid upthe draft tube toward the surface aerator.

FIG. 7B shows a partial cross-sectional view of a sixth embodiment ofthe wastewater treatment system of the present invention, showing arotatable surface aerator connected to a first driven shaft and atapered rotatable draft tube fixedly connected to a bottom portion ofthe surface aerator, and showing a rotatable submerged rotor or highefficiency turbine impeller mounted to a second driven shaft andpositioned therein the interior of the draft tube for moving liquid upthe draft tube toward the surface aerator.

FIG. 8 shows a partial cross-sectional view of a seventh embodiment ofthe wastewater treatment system of the present invention, showing arotatable surface aerator connected to a driven shaft and a rotatabledraft tube fixedly connected to a bottom portion of the surface aerator,and showing at least one impeller blade mounted to an interior surfaceof the draft tube for moving liquid up the draft tube toward the surfaceaerator.

FIG. 9 shows a partial cross-sectional view of a eighth embodiment ofthe wastewater treatment system of the present invention, showing arotatable surface aerator connected to a driven shaft and a rotatabledraft tube fixedly connected to a bottom portion of the surface aerator,and showing a source a gas in communication with the interior andexterior of the draft tube.

FIG. 10 is a chart that illustrates the time for comparative scaledmodels of aeration devices to completely aerate water in a test tank.

FIGS. 11A and 11B are charts that illustrate test velocity profiles of ascaled embodiment of the present invention having a rotatable surfaceaerator connected to a driven shaft and a rotatable draft tube fixedlyconnected to a bottom portion of the surface aerator. The distance fromthe wall is the distance from a sidewall of a model orbital tank havinga 24 inch wide channel with a water depth of 42 inches.

FIGS. 12A and 12B are charts that illustrate test velocity profiles of ascaled embodiment of a conventional system, showing the results for atypical Eimco aerator/mixer present invention having a rotatable surfaceaerator connected to a driven shaft and a rotatable submerged mixerrotor. The distance from the wall is the distance from a sidewall of amodel orbital tank having a 24 inch wide channel with a water depth of42 inches.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be understood more readily by reference to thefollowing detailed description, examples, drawings, and claims, andtheir previous and following description. However, before the presentdevices, systems, and/or methods are disclosed and described, it is tobe understood that this invention is not limited to the specificdevices, systems, and/or methods disclosed unless otherwise specified,as such can, of course, vary. It is also to be understood that theterminology used herein is for the purpose of describing particularaspects only and is not intended to be limiting.

As used in the specification and the appended claims, the singular forms“a,” “an” and “the” include plural referents unless the context clearlydictates otherwise.

Ranges may be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another embodiment includes from the one particular valueand/or to the other particular value. Similarly, when values areexpressed as approximations, by use of the antecedent “about,” it willbe understood that the particular value forms another embodiment. Itwill be further understood that the endpoints of each of the ranges aresignificant both in relation to the other endpoint, and independently ofthe other endpoint.

“Optional” or “optionally” means that the subsequently described system,component, event or circumstance may or may not occur, and that thedescription includes instances where system, component, event orcircumstance is included and instances where it is not included.

The present invention may be understood more readily by reference to thefollowing detailed description of preferred embodiments of the inventionand the examples included therein and to the Figures and their previousand following description.

In one aspect of the present invention, a wastewater treatment system 10comprises a tank 20, which comprises at least a pair of channels 21 formovement of a volume of a liquid, and an elongate draft tube 30, whichis at least partially submerged beneath the liquid. The tank or basinhas a bottom 22 and is configured to hold a predetermined volume of aliquid. In one aspect, the tank 20 or basin of the present invention isa conventional orbital tank. In one example, the orbital tank 20 has aracetrack configuration formed by an outer wall 23, at least a pair ofturning walls 24, and at least one partition wall 26. Alternatively, theracetrack configuration can be formed by the outer wall 23, additionalturning walls 24, and a series of partition walls 26. In one aspect, anend 27 of the at least one partition wall 26 extends short of a tankcurved turning wall to form the at least a pair of channels for movementof the volume of liquid. In one exemplary aspect, the at least a pair ofchannels extend substantially parallel to each other. Normally, all ofthe walls of the orbital tank are constructed of concrete. It will beappreciated that the design of the tank or orbital basin 20 is typicallybased on individual design parameters such as influent quantity andcharacteristics, desired effluent levels, and the wastewater system'ssite size and shape.

Conventionally, wastewater can reside within the tank 20 for twelve ormore hours and cycle repetitiously around the overall circuit of thetank. In one typical aspect, influent enters the system by pipe and thelevel of the mixed liquor is controlled by a conventional weir that alsofunctions to remove mixed liquor from the top surface of the system.

In operation, the draft tube 30 is rotated about its longitudinal axisfor orbitally moving the liquid through the at least a pair of channels21 of the tank in a fixed direction. In one aspect, the draft tube 30provides for propulsion of the mixed liquor in the channels of the tank.As one will appreciate, the rotation of the draft tube induces rotatingmotion in the surrounding liquid for mixing and moving the liquid in thetank channels. The viscous draft of the rotating exterior surface 32 ofthe draft tube imparts a significant rotary motion to the liquidsurrounding the draft tube 30. Thus, the rotating draft tube 30 has arotary effect on the liquid as deep as the draft tube extends. In anexemplary example and not meant to be limiting, the draft tube 30 of thepresent invention could be about 8 feet in diameter and be about 20 feetin height. This exemplary draft tube would impart about 25H.P. into theliquid at normal operative speeds.

In a further aspect, the elongate draft tube is mountable to a firstdriven rotative shaft 40 and is rotatable about the longitudinal axis ofthe first driven rotative shaft. In one example, the longitudinal axisof the draft tube is substantially co-axial with the longitudinal axisof the first driven rotative shaft. In one aspect, the longitudinal axisof the draft tube 30 extends substantially parallel to the end 27 of thepartition wall 26. In one aspect, the exterior surface 32 of the drafttube is configured to be spaced less that about 36 inches from the end27 of the partition wall. To this end, the system 10 can be configuredto operate with a spacing between the exterior surface 32 of the drafttube and the end 27 of the partition wall of 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27,28, 29, 30, 31, 32, 33, 34 and 35 inches and any range derived fromthese values. Of course it is contemplated that the exterior surface ofthe draft tube and the end of the partition wall will be substantiallyparallel to each other such the distance between the exterior surface ofthe draft tube and the end of the partition wall is substantiallyconstant. Alternatively, the distance between the exterior surface ofthe draft tube and the end of the partition wall can vary, with theclosest distance between respective portions of the exterior surface ofthe draft tube and the end of the partition wall being within the rangesoutlined above.

In another aspect, the distal end 34 of the draft tube is positioned apredetermined distance from the bottom 22 of the tank. In one example,the distal end of the draft tube is spaced from the bottom of the tank adistance less than or equal to the diameter of the opening defined inthe distal end of the draft tube. Alternatively, the predetermineddistance can be less than or equal to about ¾ of the diameter of theopening defined in the distal end of the draft tube. In another example,the predetermined distance can be less than or equal to the radius ofthe opening defined in the distal end of the draft tube. Of course, itis contemplated that the predetermined distance can be greater than orequal to the diameter of the opening defined in the distal end of thedraft tube.

In a further aspect, the wastewater system 10 can further comprise afixed axial support shaft 42 mounted to and extending therefrom thebottom 22 of tank. The system can further comprise a bearing means forrotatively supporting the distal end 34 of the draft tube. In thisaspect, the distal end of the draft tube is rotatively supported suchthat the draft tube will not oscillate about it longitudinal axis. Inoperation, the rotative shaft 40 and the fixed support shaft 42cooperate to allow for the rotation of the draft tube about itslongitudinal axis.

In a further aspect of the system, the diameter of the proximal end 36of the draft tube 30 is greater than the diameter of the distal end 34of the draft tube 30. By having a reduced inlet diameter at the distalend 34 of the draft tube, fluid is pumped from the distal end 34 of thedraft tube toward the proximal end 36 as the draft tube is rotated aboutits longitudinal axis. In one aspect, the diameter of the draft tubeincreases as the draft tube extends from the distal end towards theproximal end. It is believed that the pumping action may be a result ofcentrifugal force, due to the rotation of the draft tube, acting on theliquid against the draft tube's inner diameter. The liquid can thenequally flow up or down the draft tube 30 to leave the high pressurearea. By having the diameter of the proximal end 36 of the draft tubebeing greater than the diameter of the distal end 34 of the draft tube,the liquid is urged or directed toward the area of increasing insidediameter.

In a further exemplary aspect, the draft tube 30 may further comprise aninsert 37 that is mountable in a distal end portion of the draft tube.In this example, the insert 37 defines a central opening 38 that has adiameter that is less than the inside diameter of the proximal end 36 ofthe draft tube. Thus, in one aspect, the insert can be a substantiallyplanar ring shaped member that is mountable to the distal end 34 of thedraft tube, or alternatively, that is mountable to the interior surface33 of the draft tube 30 within a distal end portion of the draft tube.

In one example, the draft tube 30 can be substantially cylindrical. Bypartially enclosing the distal end of a cylindrical draft tube 30, suchas by use of the insert 37, the fluid can be pumped from the distal end34 toward the proximal end 36 of the draft tube.

In another aspect, the draft tube 30 has a frustoconical shape in whichthe diameter of the proximal end of the draft tube is greater than thediameter of the distal end of the draft tube. In this aspect, the insert37 can, if desired, be mounted to a distal end portion of the drafttube.

In a further aspect and referring to FIG. 3, the wastewater system 10may comprise a flexible blade 50 mounted to at least a portion of theend 27 of the partition wall 26. In operation, at least a portion of theflexible blade is in slideable contact with a portion of the exteriorsurface 32 of the rotating draft tube.

In another aspect of the invention and referring to FIG. 9, thewastewater system 10 can further comprise a plurality of impeller blades52 mountable to the interior surface 33 of the draft tube. Each impellerblade 52 is configured to move liquid up the draft tube toward theproximal end 36 of the draft tube 30 as the draft tube is rotated. Inthe exemplary example outlined above, for a draft tube 30 havingdimensions of about 8 feet in diameter and about 20 feet in height, theaddition of 4 inch by 1 inch wide blades vertically up the exemplary 20foot draft tube would increase the power transmitted into the liquid atnormal operating speeds by approximately 146H.P.

In a further aspect, the wastewater treatment system 10 furthercomprises a rotatable surface aerator 60 that is at least partiallyimmersible into the liquid. In use, the surface aerator 60 is rotatedabout its longitudinal axis to move the liquid upward and distribute itover the liquid surface. The aerator 60 acts to provide mixed liquormixing, aeration of the mixed liquor of waste water and activatedsludge, and aid in the flow of the mixed liquor through the channels ofthe tank.

In one embodiment of the present invention, the rotatable surfaceaerator 60 is mounted to a portion of the first driven rotative shaft40. In one aspect, as noted above, the rotatable surface aerator 60 isat least partially immersible into the liquid and is adapted to rotateabout the longitudinal axis of the first driven rotative shaft 40 tomove the liquid upward and distribute it over the liquid surface. In oneexample of the system 10, at least a portion of the rotative shaft 40extends into the liquid contained therein the tank. In alternativeaspects, it is contemplated that the rotative shaft 40 is mounted to anupper portion 62 of the surface aerator 60 and does not extend into theliquid contained in the tank.

In one aspect, the proximal end 36 of the draft tube 30 underlies thesurface aeration rotor 60. In a further aspect, the diameter of thesurface aerator 60 is at least equal to the diameter of the proximal end36 of the draft tube. The larger diameter of the surface aerator 60relative to the inside diameter of the proximal end 36 of the draft tube30 allows for a lower rotation speed to achieve the desire dischargewater velocity of about 10 to 25 feet per second. In another aspect, thelarger diameter of the surface aerator 60 relative to the insidediameter of the proximal end of the draft tube 30 allows for thedischarged liquid to accelerate more slowly as it moves from the insidediameter of the proximal end of the draft tube to the larger outsidediameter of the surface aerator. This allows for a reduction is powerrequired to achieve the desired discharge velocity. To this end, thesystem can be configured to operate with a desired discharge velocity of11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 and any rangederived from these values. It is, of course, contemplated that thediameter of the surface aerator 60 can be less than the diameter of theproximal end 36 of the draft tube 39.

It another aspect of the invention, the proximal end 36 of the drafttube is positioned adjacent to a bottom portion 64 of the surfaceaerator. In one example of this aspect, the proximal end of the drafttube is connected directly to the bottom portion of the surface aerator.In this exemplary aspect, the length of the draft tube would besubmerged. Alternatively, and as shown in FIG. 6, it is contemplatedthat the proximal end 36 of the draft tube is spaced a predetermineddistance from the bottom portion 64 of the surface aerator 60. In thisexemplary aspect, the length of the draft tube would be submerged.

The combination of the rotating surface aerator 60 and the underlyingrotating draft tube 30 focuses the suction effect of the aerator deep inthe tank. The allows for the construction of very deep oxidationsditches since the imparted rotary motion to the liquid contained withinthe tank would be substantially uniform from the top of the tank down tothe distal end of the rotating draft tube.

It is contemplated that the aeration efficiency of the rotor and draftassembly would be greater than the surface aerator alone. In thewastewater treatment system of the present invention, liquid flows upthrough the draft tube from its distal end to its proximal end and exitsthrough the surface aerator to be sprayed into the air at the liquidsurface. This is in contrast to conventional surface rotors that pump aportion of their water below the surface and not into the air.

In a further aspect, and referring to FIGS. 7A and 7B, the wastewatertreatment system further comprises a second driven rotative shaft 44 anda submerged rotor 70 that is mounted to the second driven shaft 44 andthat acts as an axial flow impeller. In this aspect, the second drivenrotative shaft 44 has a longitudinal axis and is positioned within atleast a portion of the draft tube 30. In one exemplary aspect, the firstrotative shaft 40 is hollow and the second rotative shaft 44 extendsthrough the hollow of the first rotative shaft. In this aspect, it iscontemplated that the first rotative shaft 40 can be connected to theupper portion 62 of the surface aerator 60 and the draft tube 30 wouldbe connected to the bottom portion 64 of the surface aerator.

In one aspect, the submerged rotor 70 is mounted to the second drivenrotative shaft 44 within the interior volume of the draft tube 30 and isconventional configured to move liquid up the draft tube toward thesurface aerator. In one aspect, the second driven rotative shaft 44 isrotated in the same direction as the draft tube at a speed at leastequal to the rotation speed of the first driven rotative shaft (i.e.,the rotative speed of the draft tube). For example, the second drivenrotative shaft 44 can be rotated at a multiplier about and between 1.1to about 3.0 of the speed of the first rotative shaft 40. Having aseparate drive 102 for the second rotative shaft 44 permits regulationof the amount of liquid that is pumped up the draft tube.

It is contemplated, in one embodiment of the invention, that therespective rotations of the surface aerator 60 and the connected drafttube 30 and the internal, submerged rotor 70 be separately controlled sothat the power to rotate or mix the liquid can be adjusted by changingthe draft tube rotation speed. In this aspect, the amount of oxygeninducted into the water can be controlled by changing the speed of theaxial flow impeller 70. Of course, it is further contemplated that, insizing the system for the specific application, the relative sizes ofthe surface aerator 60 and the connected draft tube 30 and the submergedrotor 70 can be selected to put more or less power to mixing or aerationas required by the desired application process demands.

In an alternative aspect, the, the second driven rotative shaft 44 isrotated in the opposite direction as the draft tube 30 at a select speedor is non-rotative with respect to the draft tube 30. In thisalternative aspect, and as one skilled in the art will appreciate, thepitch of the submerged rotor 70 is generally opposite to the pitch ofthe submerged rotor that rotates in the same direction as the drafttube. Here, the rotor 70 uses or recovers the power in the water beingrotated by viscous drag inside the moving draft tube 30.

Referring to FIG. 9, the wastewater treatment system of the presentinvention can also comprise a source of gas 80 that is in communicationwith a portion of the system to supplement the amount of gas that issupplied to the system. The source of gas acts to increase theefficiency of the system. In one aspect, in which the lower portion ofthe first driven rotative shaft is positioned within the draft tube, thefirst driven rotative shaft 42 defines a bore 82 in communication withat least one aperture 84 in the lower portion of the first drivenrotative shaft. The bore is in communication with the source of gas suchthat the gas can be selectively injected into the fluid within the drafttube via the at least one aperture 84.

In another aspect, the draft tube 30 defines at least one aperture 86that is in communication with the source of gas 80 for selectivelyinjecting gas into the fluid proximate the at least one aperture. In oneaspect, it is contemplated that the at least one aperture 86 can bepositioned on an interior surface 33 of the draft tube. In anotheraspect, the at least one aperture 86 can be positioned on an exteriorsurface 32 of the draft tube. In a further aspect the at least oneaperture 86 can be positioned on respective select portions of both theinterior and exterior surfaces of the draft tube. Further, it iscontemplated that the at least one aperture 86 can be positioned on anyselect portion of the upper or lower portions of the draft tube 30.

In another aspect of the invention, the wastewater treatment system 10can further comprise a drive unit 100 that is positioned above theliquid surface of the volume of liquid. In one aspect, the drive unit100 is positioned between the end 27 of one of the at least onepartition walls 26 and the tank curved turning wall 24. In one aspect,the drive unit 100 is operatively coupled to the first driven rotativeshaft.

In an exemplary example of the operation of the system of the presentinvention, the submerged elongate draft tube is rotated to orbitallymove the liquid through the at least a pair of channels of the tank in afixed direction. As the draft tube is rotated, liquid is continuouslydrawn into the distal end of the draft tube and is propelled toward theproximal end of the draft tube. As noted above, if the system furthercomprises a rotating surface aerator, the rotation of the surfaceaerator acts to aerate the liquid and to move at least an upper portionof the liquid within the tank. In one aspect, by positioning the drafttube underneath the bottom portion of the surface aerator and rotatingthe surface aerator and draft tube about a common longitudinal axis,liquid is continuously drawn into the distal end of the draft tube,through the proximal end of the draft tube, and is directed toward thebottom portion of the surface aerator by the pumping action of thesurface aerator.

EXAMPLES

To further illustrate the principles of the present invention, thefollowing examples and experimental data are put forth so as to providethose of ordinary skill in the art with a complete disclosure anddescription of how the devices, systems and methods claimed herein canbe made and evaluated. They are intended to be purely exemplary of theinvention and are not intended to limit the scope of what the inventorsregard as their invention. Efforts have been made to ensure accuracywith respect to numbers (e.g., SOTR, average velocities, etc.); however,some errors and deviations may have occurred.

Tests were conducted of 1/10 scale models of the system of the presentinvention and an exemplary DHV/Eimco aerator with a lower mixer. Theexemplary DHV/Eimco aerator with a lower mixer is of a type previouslyexemplified in the U.S. Pat. No. 4,869,818 to DiGregorio, et al, inwhich a radial flow submerged impeller is added to the same shaft thatdrives the surface aerator so that mixed liquor in the lower portion ofthe orbital channels is pumped in the same direction as that mixedliquor pumped by the surface aerator. The scaled model of the system ofthe present invention comprised a draft tube co-axially mounted to abottom portion of a surface aerator, which extended to near the testbasin floor. In both scale models that were tested, a similar low speedsurface aerator was used. The model orbital tank size was 4′ wide by 8′long by 4′ deep, with 24 inch wide channels.

FIG. 10 shows the time it took each scaled aeration device to completeaerate water is a test orbital tank. The vertical scale is mg/L ofOxygen in water and the horizontal axis is time data points at 6 secondintervals. As noted in the chart, the system of the present inventionmore efficiently aerated the liquid in the test tank.

Uniform ditch water velocity is important in deep ditch applications.Tests of the exemplified systems were conducted in which the DHV/Eimcoaerator with the lower mixer and the surface aerator and draft tubecombination of the present invention were both run in the 1/10 scaleorbital test tank at a water depth of 42 inches (which is a scaled depthof about 10 meters). Both devices were run to put substantially equalpower into the orbital test tank. Water velocities were recorded on agrid cross-section of the channel or ditch of the orbital test tank.Results of the test are illustrated in FIGS. 11A-12B. For substantiallyequal power, the system of the present invention was over 2 times moreefficient in moving water around the ditch of the orbital tank. Theaverage velocity of the system of the present invention was about 0.80fps versus about 0.34 fps for the DHV/Eimco aerator. Further, the systemof the present invention had a velocity variation from average of about85% as compared to the velocity variation of the DHV/Eimco aerator ofabout 297%.

PRESENT INVENTION VELOCITY (FT/SEC) MODEL RESULTS (in) 3 9 15 21 3 0.800.68 0.81 0.95 9 0.52 0.60 0.73 0.95 15 0.80 0.69 0.72 0.79 21 1.18 0.740.81 0.78 27 1.26 0.89 0.89 0.79 33 1.10 0.63 0.70 0.75 39 0.81 0.530.69 0.73

DVH/Eimco System VELOCITY (FT/SEC) MODEL RESULTS (in) 3 9 15 21 3 0.880.69 0.68 1.06 9 0.65 0.31 0.25 0.45 15 0.48 0.05 0.05 0.23 21 0.35 0.050.05 0.21 27 0.26 0.05 0.05 0.18 33 0.44 0.05 0.05 0.10 39 0.90 0.500.25 0.25

Oxygen transfer was evaluated using the ASCE clean water test proceduresand liquid pumpage was determined using a velocity meter. A summary ofthe test results are shown in the following table.

SUMMARY EXPERIMENTAL RESULTS DVH/Eimco INVENTIVE SYSTEM SYSTEMPARAMETERS MODEL MODEL RPM 450 445 HP_(WATER) 0.51 0.52 K_(L)a₂₀, hr⁻¹9.56 13.8 SOTR, lbO₂/HR 0.47 0.69 Avg. Velocity, ft/sec 0.34 0.80

As noted above, it can be concluded from the test results that thedesign of the system of the present invention is significantly moreefficient in both liquid pumpage and oxygen transfer. In addition, itshould be noted that the system of the present invention produced asignificantly more uniform velocity profile in both width and depth ofthe entire tank or basin when compared to the conventional DVH/Eimcodesign.

The preceding description of the invention is provided as an enablingteaching of the invention in its best, currently known embodiment. Tothis end, those skilled in the relevant art will recognize andappreciate that many changes can be made to the various aspects of theinvention described herein, while still obtaining the beneficial resultsof the present invention. It will also be apparent that some of thedesired benefits of the present invention can be obtained by selectingsome of the features of the present invention without utilizing otherfeatures. Accordingly, those who work in the art will recognize thatmany modifications and adaptations to the present invention are possibleand can even be desirable in certain circumstances and are a part of thepresent invention. Other embodiments of the invention will be apparentto those skilled in the art from consideration of the specification andpractice of the invention disclosed herein. Thus, the precedingdescription is provided as illustrative of the principles of the presentinvention and not in limitation thereof. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims.

1. A system for aerating a liquid, comprising: a tank having a bottom,wherein the tank holds a volume of the liquid; a first driven rotativeshaft having a longitudinal axis, wherein at least a portion of theshaft extends into the liquid; a rotatable surface aerator mounted tothe first driven rotative shaft and at least partially immersible intothe liquid, wherein the surface aerator is adapted to rotate about thelongitudinal axis of the first driven rotative shaft to move the liquidupward and distribute it over the liquid surface; and an elongate drafttube having a longitudinal axis, a proximal end and a distal end, thedraft tube being mountable to the first driven rotative shaft androtatable about the longitudinal axis of the first driven rotativeshaft, wherein the longitudinal axis of the draft tube is substantiallyco-axial with the longitudinal axis of the first driven rotative shaft;wherein the proximal end of the draft tube underlies the surfaceaerator, and wherein the distal end of the draft tube is positioned apredetermined distance from the bottom of the tank.
 2. The system ofclaim 1, wherein the proximal end of the draft tube is positionedadjacent a bottom portion of the surface aerator.
 3. The system of claim1, wherein the proximal end of the draft tube is spaced a predetermineddistance from a bottom portion of the surface aerator.
 4. The system ofclaim 1, wherein the draft tube is a substantially cylindrical tube. 5.The system of claim 4, further comprising an insert mountable in thedistal end of the draft tube, the insert defining a central opening thathas a diameter that is less than the diameter of the proximal end of thedraft tube.
 6. The system of claim 1, wherein the diameter of theproximal end of the draft tube is greater than the diameter of thedistal end of the draft tube.
 7. The system of claim 6, wherein thedraft tube has a frustoconical shape.
 8. The system of claim 1, whereinthe diameter of the surface aerator is at least equal to the diameter ofthe proximal end of the draft tube.
 9. The system of claim 1, furthercomprising a plurality of impeller blades mountable to an interiorsurface of the draft tube for moving liquid up the draft tube toward thesurface aerator.
 10. The system of claim 1, further comprising: a seconddriven rotative shaft having a longitudinal axis, wherein the seconddriven rotative shaft is positioned therein the elongate draft tube; anda submerged rotor mounted to the second driven rotative shaft for movingliquid up the draft tube toward the surface aerator, wherein the seconddriven rotative shaft is rotated at a speed at least equal to therotation speed of the first driven rotative shaft.
 11. The system ofclaim 1, further comprising a source of gas; wherein the lower portionof the first driven rotative shaft is positioned within the draft tube,wherein the first driven rotative shaft defines a bore in communicationwith at least one aperture in the lower portion of the first drivenrotative shaft, and wherein the bore is in communication with the sourceof gas such that the gas is injected into the fluid within the drafttube via the at least one aperture.
 12. The system of claim 1, whereinthe tank is an orbital tank having at least one partition wall extendingshort of a tank curved turning wall to form at least a pair of channelsfor movement of the volume of liquid.
 13. The system of claim 12,further comprising a drive unit positioned above the liquid surface andbetween an end of one of the at least one the partition wall and a tankcurved turning wall, wherein the drive unit is coupled to the firstdriven rotative shaft.
 14. The system of claim 12, wherein the drafttube is spaced less than about 10 inches from the partition wall. 15.The system of claim 12, wherein the longitudinal axis of the draft tubeextends substantially parallel to the partition wall.